An aqueous emulsion of PTFE is prepared by emulsion-polymerizing a tetrafluoroethylene monomer in the presence of an aqueous polymerization initiator and fluorine-containing emulsifying agent (for example, U.S. Pat. No. 2,559,752). Fluorine-containing polymers other than PTFE are also prepared by nearly the same process, and a solid content thereof is usually from 5 to 45% by weight. In case of industrial uses such as coatings and binders for batteries, an aqueous dispersion having a high concentration (for example, about 60% by weight) is demanded. However it is difficult to obtain a high concentration simply by adding an additional amount of fluorine-containing polymer, and such a high concentration is usually achieved by concentrating procedure.
For example, U.S. Pat. No. 2,478,229 discloses a method for concentrating aqueous dispersion of fluorine-containing polymer by using anionic surfactant. However that method has not been practically adopted since there is a problem that when making the surfactant insolubilized by adding a large amount of electrolyte, a part of PTFE particles causes an avoidable irreversible coagulation.
Also a method of using nonionic surfactant disclosed in U.S. Pat. No. 3,037,953 can give an aqueous dispersion having a relatively high concentration without causing coagulation of PTFE particles and has been commercialized. However the defects of that method are such that since a large amount of a specific surfactant having the limited cloud point range (commonly 20.degree. to 80.degree. C.) is used and after the concentration, the surfactant (hydrophilic portion) having a deviated molecular weight distribution has to be wasted from a supernant liquid, a large material cost is necessary in raw materials, and such that since a large heat energy consumption and long-time process steps are required, the efficiency of the process is not good.
Further in a method of concentration by evaporation disclosed in U.S. Pat. No. 3,316,201, since the resulting concentrated dispersion contains an aqueous polymerization initiator and fluorine-containing emulsifying agent at high concentration which are used in polymerization reaction, there is a defect that viscosity of the aqueous dispersion is sensitively changed with its temperature. In addition, in case of an aqueous dispersion having poor dispersion stability such as aqueous dispersion of PTFE, primary particles easily cause coagulation during the concentrating step to break emulsion state.
In an electric decantation method disclosed in GB 642,025, since the resulting coagulated particles adhere to an electrode to obstruct the passing of electric current, concentration effect is very low with a large amount of electric energy consumption. Thus that method is not suitable for practical use.
On the contrary, there is proposed a membrane separation method which employs an ultrafilter membrane (UF membrane) having a pore size up to 0.5 time the minimum particle size of fluorine-containing polymer particles (JP-B-2-34971, U.S. Pat. No. 4,369,266). This membrane separation method employing UF membrane has been used for filtration of high molecular weight materials, and also in the concentration of an aqueous dispersion of fluorine-containing polymer, has merits such as low energy consumption and low cost, use of various surfactants and a reduced amount of the surfactant.
However in the membrane separation method employing UF membrane, there are problems to be solved such that: 1 since a separable molecular weight of UF membrane is as small as from about 1,000 to hundreds of thousands (not more than 0.01 .mu.m in pore size), desired filtrating performance cannot be obtained unless filtration pressure is adjusted to about 1 MPa. Also filtration speed is low and when liquid flow rate is increased, the UF membrane is clogged with the fluorine-containing polymer particles, and 2 since an aqueous dispersion has to be fed forcedly to the UF membrane and a fluorine-containing polymer, particularly PTFE is fibrilated by mechanical shearing force, in case of using a pump having mechanically movable mechanism, the polymer is fibrilated by a shearing force generated at the moving parts or sealing parts of the pump, which causes a trouble that passage of the dispersion and the UF membrane are clogged with the resulting fibrilated product.
According to technique disclosed in JP-B-2-34971, the problem of the above 2 is intended to be solved by using a pump which keeps the fluorine-containing polymer particles off mechanical parts causing friction e.g. a peristalic pump or preferably a centrifugal pump. However even by the use of a centrifugal pump which is considered to be preferable in the above-mentioned patent publication, a shearing force is generated at rotating parts and the technique does not give substantial solution.
An object of the present invention is to provide a method for concentration which makes improvements on the problems of above 1 such as high filtrating pressure, low filtration speed and long filtration time and can solve the problem with the fibrilation of the above 2 while maintaining such merits of the UF membrane separation method that kind of surfactants is not limited and an amount of the surfactant can be decreased, and further improving its low energy consumption and low cost.